If she's reading the mail with OE - it's possible she's got it set to display only unread mails. Check in OE under View - Current view.
A bit more info on the mail-client/whether webmail or not - would help.
Fujitsu Lifebook P727 laptop review
Converse's new logo: the trainer brand looks to its heritage for a fresh identity
Mac power user tips and hidden tricks
Comment lancer Windows 10 en mode sans échec ?